1. Field of the Invention
The present invention relates to the halftone processing of grayscale image data by an image processing apparatus such as a laser printer. More specifically, the present invention relates to halftone processing that uses predetermined cells (pixel groups) to find the position of a reference point from the grayscale values of the pixels contained in a cell and generates dots (quantization data) in a proportion corresponding with the sum of the grayscale values of the pixels in the cell in the position of the reference point.
2. Description of the Related Art
Image processing apparatuses such as printers conventionally perform printing on print paper by converting grayscale data with multiple-value grayscale values for each pixel to binary values representing the existence of a dot. Generally, the processing to convert the multiple-value grayscale values to binary values is known as halftone processing.
Such halftone processing includes processing that searches for pixels in a predetermined order until predetermined grayscale values have accumulated and redistributes the accumulated grayscale values in order starting with the pixel with the largest input grayscale values (CAPIX hereinbelow) (Kurosawa, Toshiya and 3 others; “Peripheral brightness accumulation redistribution” (CAPIX method) pseudo intermediate grayscale processor”, Institute of Image Electronics Engineers of Japan, 17th Volume, Fifth Issue (1988), for example).
Furthermore, pixel groups (‘cells’ hereinbelow) consisting of a plurality of pixels are constituted by selecting pixels in a predetermined order until the total of the grayscale values of the pixels reaches a predetermined threshold value, and halftone processing to generate dots in the center position of the cell is also known (Japanese Patent Application Laid Open No. H11-27528, for example).
However, when pixels with a large input grayscale value exist in a scattered configuration in (Kurosawa, Toshiya and 3 others; “Peripheral brightness accumulation redistribution” (CAPIX method) pseudo intermediate grayscale processor”, Institute of Image Electronics Engineers of Japan, 17th Volume, Fifth Issue (1988)), the accumulated grayscale values are distributed in accordance with the input grayscale values and the dots also exist in a scattered configuration. When the isolated dots are reproduced by a laser printer, this does not necessarily means that stabile dots can be reproduced. This is because, with regard to the adhesion of toner to the light-sensitive drum, dots of a large size are more stable. Such unstable dots are a cause of image quality deterioration.
Further, Japanese Patent Application Laid Open No. H11-27528 constitutes cells by selecting pixels until the total of the grayscale values is “255” and generates dots at the pixels located at the center of the cells. Therefore, because a dot of one pixel is generated, isolated dots are similarly produced and dot reproduction is unstable.